Hair cells of the auditory and vestibular systems signal sensory stimuli as graded changes in neurotransmitter release and employ unique anatomical and molecular components that differ from conventional synapses. Among the unique molecular features is the apparent lack of reliance on neuronal SNARE proteins and their various partners. Instead, hair cell exocytosis depends on a large protein called Otoferlin, by unknown mechanisms. In this proposal, we investigate the unique features of hair cell synaptic transmission using a combination of molecular biology, electrophysiological approaches in zebrafish, and novel in vitro membrane fusion assays. In Aim 1 we will determine whether Otoferlin can stimulate membrane fusion using well established cell-cell fusion assays with engineered cells and determine the requirements for SNAREs, lipids and calcium in otoferlin-dependent fusion. These experiments will also determine the functional domains required to mediate membrane fusion. Importantly, this strategy avoids difficulties with purification of Otoferlin that hindered advances in reconstituting Otoferlin-dependent fusion in the past. In Aim 2, we will measure the calcium-dependent membrane binding properties of Otoferlin and look for Otoferlin-interacting partners in native cells. In Aim 3, we use the zebrafish lateral line as a model system explore the role of SNAREs in hair cell exocytosis. In Aim 4, we test the effectiveness of truncation mutants to rescue synaptic function. Understanding hair cell synaptic function at the molecular level will ultimately aid in understanding how auditory information is processed and communicated. Moreover, mutations in Otoferlin lead to DFNB9 form of inherited deafness and thus study of its function has relevance for human disease. Otoferlin belongs to the ferlin class of proteins, which include myoferlin and dysferlin, which are also implicated in membrane fusion and human disease. It is reasonable to expect that what we learn in this project will be instructive for studies with other ferlins. Lastly, the fundamental understanding of presynaptic processes in these specialized cells will have broader implications for cellular communication in general and thus, may contribute to our understanding of various aspects of mental health and neurological disorders.